Giant clams belong to the Mollusk group, which is the most diverse group of lophotrochozoans. The mollusk group consists of about 100,000 species that inhabit a wide array of aquatic and terrestrial environments.
There are four major clades of mollusks:
Chitons: There are approximately 1,000 living species of chitons. These are characterized by eight overlapping calcareous plates surrounded by a structure referred to as the girdle. The plates and the surrounding girdle protect its internal organs and its muscular foot.
Gastropods are the most species-rich and widely distributed mollusks with nearly 70,000 living species. These include snails, whelks, limpets, slugs, sea slugs, and abalones. Most gastropods glide on their muscular foot for movement, but some species like sea-butterflies and heteropods use the foot as a swimming organ.
Bivalves consist of clams, oysters, scallops, and mussels, as well as around 30,000 other living species. They generally share characteristics such as a small head and a two-part shell that extends fully over the sides of the body. Bivalves feed by taking water through an opening called an incurrent siphon and filtering with their large gills. Water and gametes exit through the excurrent siphon, as bivalves generally fertilize in open water.
Cephalopods appeared early in the Cambrian period. There are around 800 species of cephalopods: Squids, octopuses, and nautiluses. Each are able to control the water content of the mantle cavity by utilizing their excurrent siphon. This allows cephalopods to use the excurrent siphon as a jet-propulsion system for transport. Cephalopods capture prey using tentacles.
All four of these major clades share three similar characteristics:
A molluscan foot: a muscular structure that originally was both an organ of locomotion and a support for internal organs.
All major internal organ systems are centered in an internal visceral mass.
Mantle: a fold of tissue covering the organs of the visceral mass which secretes the calcareous shell that is typical of many mollusks.
Relationship to humans
Giant clams are preyed upon by humans who catch giant clams as a source of food. This excessive harvesting of giant clams has led to rapid decline in the worldwide giant clam population (1).
The adductor muscle of the clams is considered a delicacy; however, over-harvesting of the species for consumption, for shells and for aquarium trade has resulted in its climb to the top one group's "vulnerable" list. (7). In addition, giant clams are an interest for humans because they produce black pearls, worth thousands of dollars (14).
Habitat and niche
The giant clam is an aquatic organism which feeds by filtering water through its gills which it takes in via a structure called an incurrent siphon. Giant clams are most typically found in the coral reefs of the Indian and South Pacific oceans. Once the giant clam has found an acceptable location on a reef, it anchors there and remains in that spot for the rest of its life which can last more than 100 years (1).
Predator avoidance
The hard shell acts as a physical barrier protecting the muscular foot and internal organs from predators. Additionally, the giant clam avoids potential threats by embedding itself in sand or coral rubble (15).
Nutrient acquisition The giant clam's incurrent siphon brings water to the gills where gas and nutrient exchange occur. It also consumes a a great deal of sugars and proteins that are produced by the algae that live on the giant clams (1). The, "symbiotic relationship with zooxanthellae algae" gives Giant clams another source of food and energy, and in return the Giant clam protects the photosynthetic zooxanthellae algae (3). In order for this symbiotic relationship to work, the Giant clam must be in shallow water in order to catch the light.
Reproduction and life cycle (9)
Reproduction is based on fertilization which generally takes place in open water. The giant clam excretes its gametes, as well as water, from the excurrent siphon. Giant clams have both male and female reproductive organs. After releasing egg and sperm into the water, the eggs will be fertilized by the sperm of another giant clam. After fertilization, the eggs float in the water for about 12 hours until the larvae hatches out. The larvae then form a shell within a few days and find somewhere to settle on the sea floor (1).
Growth and development
The fertilized egg floats in the sea for about 12 hours until eventually a larva hatches and then starts to produce a calcium carbonate shell. The first body part it develops is a foot that helps the clam move around until it finds a good habitat to live in.
Integument
The integument of the giant clam is composed of a hard, calcareous shell which is secreted by the mantle.
The mantle is comprised of soft tissue in two loose flaps. Cells in the mantle have the ability to extract chemicals in the water and produce sticky secretions that harden to add more layers to the shell. The grooves in the shell show where the new extensions formed to give the growing clam more room. The shell is very tough and is responsible for making sure the inner organs of the clam do not disintegrate in the water. The inner layer of the shell is smooth to prevent scratches to the clam’s body (13).
Movement
All mollusks move by gliding on a muscular foot over a surface. Many clams also use their foot to burrow into mud and sand.
Adult giant clams are sessile, meaning they are immobile and cannot move themselves from the coral reef. Fertilized eggs develop into trochophores which have the capacity to swim. The swimming stage is followed by the planktonic stage, where they roam the open ocean for around one week before they settle into the substrate. (16)
Sensing the environment
Clams do not have a brain. The clam has ganglion cells that transmit information about the environment. Chemoreceptor) cells that are sensitive to the taste and movement of water detect environmental stimulus and communicate with ganglion cells. The ganglion cells receive this information from the chemoreceptor cells and perceive the motion or stimulus (6).
Gas exchange
Giant clams and all other bivalves utilize large gills for gas exchange, as well as the incurrent siphon to bring water from the outside to the gills. This gill tissue is highly vascularized, meaning it contains many blood vessels, and takes up oxygen from the water while releasing carbon dioxide.
Waste removal
Following the intake of food by the the radula, a thin layer used as the teeth for a mollusk, the food continues down the digestive track, reaching the digestive glands and eventually the intestine. Waste then exits through the anus (and enters in the mouth).
Environmental physiology (temperature, water and salt regulation)
Giant clams are coldblooded and therefore maintain a body temperature that is equal to the temperature of the immediate surroundings. The ideal temperature condition for clams is roughly around 68°F, and any temperature 20° above or below that ideal temperature is inhabitable (12).
Internal circulation
Blood and other fluids empty into a large, fluid-filled hemocoel, through which fluids move around the animal and deliver oxygen to the internal organs. The fluids eventually reenter the blood vessels and are moved by a heart.
Chemical control
Mollusks' blood salinity varies depending on the salinity of the surrounding water. In order to maintain function of enzymes, mollusks must keep ion concentration relatively stable. This is difficult when salinity decreases because the salinity of the blood will fall below the salinity of the cells, causing water to osmose from the blood into cells. This influx of water causes the cells to swell and dilutes ion concentrations, inhibiting metabolic enzyme function. It also becomes difficult with increasing salinity of surrounding water, which causes water to osmose out of the cells that have lower salinity than the blood. This efflux causes shrinkage, increasing ion concentration and disrupting proper metabolic enzyme function. (17)
Questions:
1. Describe the relationship with zooxanthellae algae: is it helpful or harmful? How does it work?
Classification/Diagnostic characteristics
(10) (11)
Domain: Eukarya
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Veneroida
Family: Tridacnidae
Genus: Tridacna
Species: Tridacna Gigas (Giant clam)
Giant clams belong to the Mollusk group, which is the most diverse group of lophotrochozoans. The mollusk group consists of about 100,000 species that inhabit a wide array of aquatic and terrestrial environments.
There are four major clades of mollusks:
- Chitons: There are approximately 1,000 living species of chitons. These are characterized by eight overlapping calcareous plates surrounded by a structure referred to as the girdle. The plates and the surrounding girdle protect its internal organs and its muscular foot.
- Gastropods are the most species-rich and widely distributed mollusks with nearly 70,000 living species. These include snails, whelks, limpets, slugs, sea slugs, and abalones. Most gastropods glide on their muscular foot for movement, but some species like sea-butterflies and heteropods use the foot as a swimming organ.
- Bivalves consist of clams, oysters, scallops, and mussels, as well as around 30,000 other living species. They generally share characteristics such as a small head and a two-part shell that extends fully over the sides of the body. Bivalves feed by taking water through an opening called an incurrent siphon and filtering with their large gills. Water and gametes exit through the excurrent siphon, as bivalves generally fertilize in open water.
- Cephalopods appeared early in the Cambrian period. There are around 800 species of cephalopods: Squids, octopuses, and nautiluses. Each are able to control the water content of the mantle cavity by utilizing their excurrent siphon. This allows cephalopods to use the excurrent siphon as a jet-propulsion system for transport. Cephalopods capture prey using tentacles.
All four of these major clades share three similar characteristics:Relationship to humans
Giant clams are preyed upon by humans who catch giant clams as a source of food. This excessive harvesting of giant clams has led to rapid decline in the worldwide giant clam population (1).
The adductor muscle of the clams is considered a delicacy; however, over-harvesting of the species for consumption, for shells and for aquarium trade has resulted in its climb to the top one group's "vulnerable" list. (7). In addition, giant clams are an interest for humans because they produce black pearls, worth thousands of dollars (14).
Habitat and niche
The giant clam is an aquatic organism which feeds by filtering water through its gills which it takes in via a structure called an incurrent siphon. Giant clams are most typically found in the coral reefs of the Indian and South Pacific oceans. Once the giant clam has found an acceptable location on a reef, it anchors there and remains in that spot for the rest of its life which can last more than 100 years (1).
Predator avoidance
The hard shell acts as a physical barrier protecting the muscular foot and internal organs from predators. Additionally, the giant clam avoids potential threats by embedding itself in sand or coral rubble (15).
Nutrient acquisition
The giant clam's incurrent siphon brings water to the gills where gas and nutrient exchange occur. It also consumes a a great deal of sugars and proteins that are produced by the algae that live on the giant clams (1). The, "symbiotic relationship with zooxanthellae algae" gives Giant clams another source of food and energy, and in return the Giant clam protects the photosynthetic zooxanthellae algae (3). In order for this symbiotic relationship to work, the Giant clam must be in shallow water in order to catch the light.
Reproduction and life cycle
Reproduction is based on fertilization which generally takes place in open water. The giant clam excretes its gametes, as well as water, from the excurrent siphon. Giant clams have both male and female reproductive organs. After releasing egg and sperm into the water, the eggs will be fertilized by the sperm of another giant clam. After fertilization, the eggs float in the water for about 12 hours until the larvae hatches out. The larvae then form a shell within a few days and find somewhere to settle on the sea floor (1).
Growth and development
The fertilized egg floats in the sea for about 12 hours until eventually a larva hatches and then starts to produce a calcium carbonate shell. The first body part it develops is a foot that helps the clam move around until it finds a good habitat to live in.
Integument
The integument of the giant clam is composed of a hard, calcareous shell which is secreted by the mantle.
The mantle is comprised of soft tissue in two loose flaps. Cells in the mantle have the ability to extract chemicals in the water and produce sticky secretions that harden to add more layers to the shell. The grooves in the shell show where the new extensions formed to give the growing clam more room. The shell is very tough and is responsible for making sure the inner organs of the clam do not disintegrate in the water. The inner layer of the shell is smooth to prevent scratches to the clam’s body (13).
Movement
All mollusks move by gliding on a muscular foot over a surface. Many clams also use their foot to burrow into mud and sand.
Adult giant clams are sessile, meaning they are immobile and cannot move themselves from the coral reef. Fertilized eggs develop into trochophores which have the capacity to swim. The swimming stage is followed by the planktonic stage, where they roam the open ocean for around one week before they settle into the substrate. (16)
Sensing the environment
Clams do not have a brain. The clam has ganglion cells that transmit information about the environment. Chemoreceptor) cells that are sensitive to the taste and movement of water detect environmental stimulus and communicate with ganglion cells. The ganglion cells receive this information from the chemoreceptor cells and perceive the motion or stimulus (6).
Gas exchange
Giant clams and all other bivalves utilize large gills for gas exchange, as well as the incurrent siphon to bring water from the outside to the gills. This gill tissue is highly vascularized, meaning it contains many blood vessels, and takes up oxygen from the water while releasing carbon dioxide.
Waste removal
Following the intake of food by the the radula, a thin layer used as the teeth for a mollusk, the food continues down the digestive track, reaching the digestive glands and eventually the intestine. Waste then exits through the anus (and enters in the mouth).
Environmental physiology (temperature, water and salt regulation)
Giant clams are coldblooded and therefore maintain a body temperature that is equal to the temperature of the immediate surroundings. The ideal temperature condition for clams is roughly around 68°F, and any temperature 20° above or below that ideal temperature is inhabitable (12).
Internal circulation
Blood and other fluids empty into a large, fluid-filled hemocoel, through which fluids move around the animal and deliver oxygen to the internal organs. The fluids eventually reenter the blood vessels and are moved by a heart.
Chemical control
Mollusks' blood salinity varies depending on the salinity of the surrounding water. In order to maintain function of enzymes, mollusks must keep ion concentration relatively stable. This is difficult when salinity decreases because the salinity of the blood will fall below the salinity of the cells, causing water to osmose from the blood into cells. This influx of water causes the cells to swell and dilutes ion concentrations, inhibiting metabolic enzyme function. It also becomes difficult with increasing salinity of surrounding water, which causes water to osmose out of the cells that have lower salinity than the blood. This efflux causes shrinkage, increasing ion concentration and disrupting proper metabolic enzyme function. (17)
Questions:
1. Describe the relationship with zooxanthellae algae: is it helpful or harmful? How does it work?
References:
1. http://a-z-animals.com/animals/giant-clam/
2. http://www.theepochtimes.com/n2/science/science-in-pics-giant-clams-60867.html
3. http://www.stanford.edu/group/microdocs/solarclams.html
4. http://www.esu.edu/~milewski/intro_biol_two/lab__11_mollusca/Mollusca.html
5. http://www.sciencedirect.com/science/article/pii/0044848681900405
6. http://www.britannica.com/EBchecked/topic/67293/bivalve/35745/The-shell
7. http://animals.nationalgeographic.com/animals/invertebrates/giant-clam/
8. http://universe-review.ca/option2.htm
9. http://www.aquasearch.net.au/aqua/clamculture.htm
10.http://www.iucnredlist.org/details/22137/0
11. http://a-z-animals.com/animals/giant-clam/
12. http://www.cooswatershed.org/Publications/clams%20and%20water%20quality.pdf
13. http://news.google.com/newspapers?nid=1499&dat=19620613&id=SjMoAAAAIBAJ&sjid=6SYEAAAAIBAJ&pg=6794,1264779
14. http://www.oceanlink.info/biodiversity/ask/mollusca.html
15. http://animaldiversity.ummz.umich.edu/accounts/Tridacna_gigas/
16. http://www.arkive.org/giant-clam/tridacna-gigas/
17. http://edis.ifas.ufl.edu/fa128